Metalloporphyrins serve as the active sites in numerous proteins that carry out important biological functions. Examples of these systems include hemoglobin (ligand binding and release). cytochromes (electron transport), photosynthetic reaction centers (charge separation), and catalase, peroxidases, and cytochrome P450 (redox and catalytic chemistry). A crucial long-standing issue is how the molecular architecture of the porphyrin and its immediate protein environment influences function. The key link between chemical composition and physical structure, on the one hand, and reaction dynamics and function, on the other hand, is the electronic structure of the metalloporphyrin. The relationships between physical structure, electronic structure, and transient state behavior are difficult to study in vivo. A major goal of our research is to investigate these relationships through time- resolved absorption studies of metalloporphyrin excited state and transient state dynamics in vitro. To this end, femtosecond and slower time scale transient absorption studies are proposed in four major areas: 1) mechanisms of radiationless deactivation of nonluminescent porphyrins, 2) mechanisms of photodissociation and photoassociation of ligands from transition metal porphyrins, 3) photophysics of porphyrin dimer sandwich complexes, and 4) excited state dynamics of porphyrin cations and anions. The underlying objectives in each area include the elucidation of the most important contributors to the electronic and vibrational factors that control the energies, rates of formation and decay, and reaction channels of the ring (Pe, Pi*) excited states, metal ring (d, Pi*) and (Pi, d) charge transfer (CT) excited states, and metal (d,d) excited states. The photophysical and photochemical behavior will be investigated for simple systems encompassing well- chosen variations in the metal ion and its oxidation state, axial ligands, porphyrin macrocycle, solvent, and temperature. The proposed studies will not only increase our understanding of metalloporphyrin transient state dynamics in vivo and in vitro, but also address issues of importance in other areas of chemistry and molecular physics, such as excited state relaxation processes and electron transfer mechanisms.